In lithographic printing, ink receptive regions, known as image areas, are generated on a hydrophilic surface. When the surface is moistened with water and ink is applied, the hydrophilic regions retain the water and repel the ink, and the ink receptive regions accept the ink and repel the water. The ink is transferred to the surface of a material upon which the image is to be reproduced. Typically, the ink is first transferred to an intermediate blanket, which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
Imageable elements useful as lithographic printing plate precursors typically comprise a top layer applied over the hydrophilic surface of a substrate. The top layer typically includes one or more radiation-sensitive components, which may be dispersed in a suitable binder. Alternatively, the radiation-sensitive component can also be the binder material. If, after imaging, the imaged regions of the top layer are removed in the developing process revealing the underlying hydrophilic surface of the substrate, the precursor is positive working. Conversely, if the unimaged regions are removed by the developing process, the precursor is negative-working. In each instance, the regions of the top layer (i.e., the image areas) that remain are ink-receptive, and the regions of the hydrophilic surface revealed by the developing process accept water and aqueous solutions, typically a fountain solution, and repel ink.
Imaging of the imageable element with ultraviolet and/or visible radiation is typically carried out through a mask, which has clear and opaque regions. Imaging takes place in the regions under the clear regions of the mask but does not occur in the regions under the opaque regions of the mask. The mask is usually a photographic negative of the desired image. If corrections are needed in the final image, a new mask must be made. This is a time-consuming process. In addition, the mask may change slightly in dimension due to changes in temperature and humidity. Thus, the same mask, when used at different times or in different environments, may give different results and could cause registration problems.
Direct digital imaging of imageable elements, which obviates the need for imaging through a mask, is becoming increasingly important in the printing industry. Thermally imageable elements have been developed for use with infrared lasers. Two different regions of the infrared spectrum are typically used for imaging: 800 nm±50 nm and 1050 nm±50 nm. Suitable commercially available imaging devices, for example, have lasers that emit at about 830 nm, at about 1056 nm, or at about 1064 nm.
Imageable elements designed for use with infrared lasers contain infrared absorbing compounds, known as photothermal conversion materials, that absorb infrared radiation and convert it to heat. Typically, these materials are effective absorbers in one of these two spectral regions, but not in both spectral regions. Imageable elements that contain these materials are sensitive to radiation in one these regions but cannot be readily imaged by imaging devices that produce radiation in the other spectral region. Consequently, different products must be manufactured for use with different types of imaging devices. In addition, the infrared absorbing compounds typically have counter ions that contain halogen ions and/or heavy metals, such as arsenic or antimony, materials that can cause environmental contamination.
Thus, a need exists for thermally imageable elements that can be readily imaged either with 800 nm±50 nm radiation or with 1050 nm±50 nm radiation and do not contain materials that can cause environmental contamination.